Clinical evaluation of an instrument to measure carbon dioxide tension at the oxygenator gas outlet in cardiopulmonary bypass

Abstract

This paper presents the clinical testing of a new capno-graph designed to measure the carbon dioxide tension at the oxygenator exhaust outlet in cardiopulmonary bypass (CPB). During CPB, there is a need for reliable, accurate and instant estimates of the arterial blood CO₂ tension (P_aCO₂) in the patient. Currently, the standard practice for measuring P_aCO₂ involves the manual collection of intermittent blood samples, followed by a separate analysis performed by a blood gas analyser. Probes for inline blood gas measurement exist, but they are expensive and, thus, unsuitable for routine use. A well-known method is to measure P_exCO₂, ie, the partial pressure of CO₂ in the exhaust gas output from the oxygenator and use this as an indirect estimate for P_aCO₂.

Based on a commercially available CO₂ sensor circuit board, a laminar flow capnograph was developed. A standard sample line with integrated water trap was connected to the oxygenator exhaust port. Fifty patients were divided into six different groups with respect to oxygenator type and temperature range. Both arterial and venous blood gas samples were drawn from the CPB circuit at various temperatures.

Alfa-stat corrected pCO₂ values were obtained by running a linear regression for each group based on the arterial temperature and then correcting the P_exCO₂ accordingly. The accuracy of the six groups was found to be (±SD): ±4.3, ±4.8, ±5.7, ±1.0, ±3.7 and ±2.1%. These results suggest that oxygenator exhaust capnography is a simple, inexpensive and reliable method of estimating the P_aCO₂ in both adult and pediatric patients at all relevant temperatures.

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